In the treatment of diseased or damaged human knee joints, particularly in cases of deterioration resulting from arthritis, surgical repair involving introduction of an artificial knee prosthesis is often required. Knee prostheses currently in use generally fall within one of two categories. First, in cases of relatively mild deterioration of the knee joint, condylar surface prostheses such as the ones disclosed in U.S. Pat. Nos. 3,728,742 and 3,774,244 are utilized. These prostheses comprise spaced pairs of artificial femoral and tibial condylar bearing portions adapted for mutual articulation. They afford substantial rotational and translational freedom of movement, are relatively light in weight, require only minor resection of natural bone for implantation, and do not require severance of the cruciate ligaments. The second category consists of hinge and ball-in-socket type devices which provide a direct linkage between the femoral and tibial components of the prosthesis. These devices are capable of providing considerable stability in cases where severe deterioration of the knee joint has occurred, but are considerably heavier and more restrictive than the prostheses of the first category and require greater resection of natural bone for implantation. Examples of prostheses in this second category include those disclosed in U.S. Pat. Nos. 3,837,009 and 3,868,730.
In cases of intermediate deterioration and instability, a need exists for a knee prosthesis that will combine the desirable features of the two categories of prostheses discussed above, i.e., the considerable freedom of rotational and translational movement, minimal resection of natural bone for implantation, and relatively light weight of the first category with the substantial degree of stability against undesired movement and dislocation provided by the linked type of prostheses.
Additionally, with knee prostheses in general, it is desirable to reduce the contact stresses and wear of the bearing portions upon each other. Excessive stress can distort the shape of the bearing portions, while excessive wear can cause the emission of significant quantities of prosthetic materials into the physiological system [see Swanson, S. and Freeman, M., "The Scientific Basis of Joint Replacement", John Wiley and Sons, Chapter 3 (1977)].
U.S. Pat. No. 3,840,905 discloses a prosthetic knee joint wherein the femoral and tibial components possess approximately saddle shapes, with the two components contacting one another in a substantially load-bearing intercondylar portion.